The present embodiments relate to a method for monitoring an exposure experienced by a patient during an examination with a magnetic resonance device.
In magnetic resonance imaging, an excitation signal may be emitted via a high-frequency coil and excites nuclear spins aligned in a basic magnetic field. When this excitation decays, a magnetic resonance signal, from which a magnetic resonance image is generated, may be observed. High-frequency radiation generated using a corresponding transmitter device of the magnetic resonance device is used for the excitation.
The high-frequency radiation (e.g., a B1 field) is absorbed in the body of the patient under examination and is converted into heat. This absorption may be described by the specific absorption rate (SAR) in relation to the patient's mass. Various standardization bodies lay down rules for monitoring the SAR for correspondingly powerful magnetic resonance devices in order to counter harm to the patient caused by too high a power being input.
Magnetic resonance devices may use a single-channel high-frequency-power amplifier in a transmitter device (e.g., circularly polarized high-frequency coils with two power supply terminals), the amplitude relationships and phase relationships being constant at both power supply terminals (e.g., identical amplitudes and 90° phase difference in the case of circular polarization). Magnetic resonance devices, in which the high-frequency coil may be actuated via several transmission channels, have been proposed. A power amplifier is associated, for example, with each of the transmission channels, the amplitudes and phases for the various transmission channels or power supply terminals being freely selectable. Embodiments, in which the number of power amplifiers does not correspond to the number of transmission channels (e.g., one power amplifier supplies at least two transmission channels via a corresponding interconnection, or one combined power supply is provided with several of the at least two power amplifiers), may also be provided.
The high-frequency power absorbed in the patient is determined asPabs=Ptx−Ploss,where Pabs describes the power absorbed in the patient, Ptx describes the transmission power, and Ploss describes the coil power loss. The specific absorption rate (SAR) is thenSAR=Pabs/m where m corresponds to the mass of the irradiated region of the patient.
When single-channel transmitter devices are used, the coil power loss stands in a fixed relationship to the transmission power, so that the SAR may be formulated, as follows:SAR=k*Ptx.The factor k includes the efficiency and the mass.
For such single-channel transmitter devices, SAR monitoring and, consequently, monitoring of the patient's exposure during an examination may be enabled using broadband power measurement devices between the power amplifier and the power supply terminals (e.g., the high-frequency coil). Broadband may be that the measurement, in which the real-value amplitude of the voltage may be measured, is not limited to a particular frequency band (e.g., a frequency band around the magnetic resonance frequency), but all frequencies may be measured. Broadband measuring devices of this type enable the transmission amplitude (e.g., a voltage amplitude) to be determined, from which the overall transmitted power follows, Ptx. With knowledge of the factor k and with correspondingly fast scanning, online monitoring during the transmission procedures may be provided.
In multi-channel transmission (e.g., parallel transmission, pTX), the factor k is no longer constant, but depends on the respective amplitude relationships and phase relationships. Consequently, a single measurement of transmission amplitudes is no longer sufficient.
In order to implement monitoring of patient exposure, broadband measuring devices may still be used and fixed factors for the amplitude relationships and phase relationships that are most harmful with respect to the specific absorption rate may be provided. This is disadvantageous in that performance is severely restricted if the actual coil power loss is significantly larger than the minimum possible that is to be applied without knowledge of the specific amplitude relationships and phase relationships for complying with SAR limit values.
In one embodiment, narrowband measuring devices that also supply phase information about the voltage as measured values in addition to the transmission amplitude may be used. The excitation pulses that are to effect the high-frequency excitation of the nuclear spins may be calculated (e.g., in a complex simulation), and a setpoint/actual-value comparison for online monitoring may be performed. The disadvantage of this variant is that independent SAR monitoring online by the actuation signal (e.g., the previously calculated excitation pulses) is no longer possible. Instead, only fixed, previously calculated pulse shapes with a prior SAR check may be used. Pulses other than the previously calculated excitation pulses may not be used, even if the pulses would not themselves cause the SAR to be exceeded. A long, safety-related chain is provided for SAR monitoring (e.g., the SAR prior calculation for each excitation pulse, the transfer of large amounts of data for the pulse shapes to a monitoring device, the comparison of the emitted excitation pulses with the expected excitation pulses, and complex error analyses in the case of small deviations from the setpoint signal).
DE 10 2008 063 630 A1 relates to a method for controlling a high-frequency transmitter device of a magnetic resonance tomography system with a transmitting antenna system with a plurality of transmission channels during a magnetic resonance measurement of an examination subject. A reference scatter parameter matrix of the transmitting antenna system is determined in the non-loaded state, and a subject-specific scatter parameter matrix of the transmitting antenna system is determined in a state loaded with the examination subject. Transmission amplitude vectors are determined on a time-dependent basis, and represent the high-frequency voltage amplitudes on the individual transmission channels. On the basis of the subject-specific scatter parameter matrix, the reference scatter parameter matrix and the transmission amplitude vectors, absorbed high-frequency power values are determined in the examination subject at particular transmission times and based on a plurality of the determined high-frequency power values a number of control values are formed. The high-frequency transmitter device is deactivated whenever a control value exceeds a predefined limit control value.